Oldenburg was the seat of the counts of Oldenburg, and received city rights in the 13th century. The oldest seal dates from the 13th century and is known since 1307. The seal shows only a castle. In the late 14th century, the patron saint, St. Lawrence is shown in the gate, with the arms of the counts of Oldenburg on each side. From the 16th century onwards the patron saint was removed and only the (simplified) arms of the counts are seen in the gate. These arms were officially adopted in 1927.

I just happened to find a photo on this webpage showing an Oldenburg flag and one of Cornwall. The flag of Oldenburg is yellow with two red stripes and the arms in the centre. Curiously, though, the arms are rotated
by 90 degrees.